The present invention relates to stimulation using electrical impulses and, in particular, to a system and method of providing electrical stimulation via the skin of a patient, by means of an inventive surface electrode.
Electrodes which are used to deliver electrical stimulation through the surface of the skin generally require the use of a conductive liquid or solid gel, often termed “hydrogel”, to provide a continuous conductive path between the skin and the current source. Conductive gels contain a salt (typically KCl or NaCl) in order to achieve the requisite electrical current flow. The preferred gel is one with a high salt content, since such a gel produces a better conductor than that obtained when using a gel with a low salt content. In addition, the use of a high salt content gel typically requires less skin abrasion at the time of application to reduce the impedance of the skin-electrode interface after subsequent electrode application.
For ease of use, it is desirable to apply the conductive liquid or solid gel at the point of manufacture, creating a “pre-gelled” electrode. U.S. Pat. No. 4,559,950 issued to Vaughn and U.S. Pat. No. 5,309,909 issued to Gadsby describe such electrodes. Pre-gelled electrodes save the step of manually applying the gel to the electrode at the time of electrode application and speed the application process considerably.
Known gels are typically hydrophilic, to improve conductivity of the gel, and perhaps more importantly, to slow the gradual dehydration of stored, sealed electrodes. It is reported by U.S. patent application No. 20020117408 to Solosko, et al., that the shelf life of an electrode pad is largely determined by the length of time it takes for enough water moisture to evaporate out of the hydrogel and escape the package of the pad. It is further articulated that as moisture escapes from the packaging, the electrical properties of the electrode pads become increasingly compromised.
This problem is a critical one for numerous and varied medical applications. For example, when electrode pads are utilized with a defibrillator, a very significant factor includes changes in small and large signal impedance values between a patient and a defibrillator. As the hydrogel dries out, the impedance values increase, making it more difficult to monitor electrical signals from the patient, obtain transthoracic impedance, and deliver energy into the body.
Water loss can affect the mechanical properties of the hydrogel as well. In some hydrogels, the loss of water causes the hydrogel to skin over or solidify, especially around the edges, which inhibits the ability of the hydrogel to adhere to the skin. This partial or complete loss of adhesion can render an electrode useless since it cannot then create or maintain an effective contact with the patient's skin. Thus, water loss from the electrode pad can prevent or attenuate receipt of electrocardiogram (ECG) signals by a defibrillator. In addition, water loss from the electrode pad can alter the delivery of defibrillation energy from a defibrillator to the patient.
Additionally, poor or uneven contact of the electrode pad with the skin of a patient may unduly concentrate energy transfer during defibrillation into areas that exhibit good skin contact. Higher than usual current densities resulting from poor or uneven skin contact can cause skin burns. If the current is not delivered to a patient in the manner for which the electrode pad was designed, the resulting treatment delivered to the patient may be altered, compromising patient outcome.
Although highly hydrophilic hydrogels slow the gradual dehydration of stored, sealed electrodes, and also slow the gradual dehydration of electrodes on most exposed skin surfaces, the changes in mechanical and electrical properties over the long term, exceed the tolerances in many medical applications. Moreover, highly hydrophilic gels have distinct disadvantages in applications requiring long-term, “wet” contact between electrode and skin, e.g., in a closed environment underneath a cast. In such wet environments, hydrophilic gels absorb water and/or sweat on the skin surface, causing swelling and even disintegration of the conductive pad.
There are several known devices for electrical stimulation of injured tissue situated underneath a cast. U.S. Pat. No. 4,574,809 to Talish, et al., entitled “Portable Non-Invasive Electromagnetic Therapy Equipment”, teaches a cast-embeddable coil structure which includes a single connector fitting, designed for exposure externally of a completed cast and for removable mounting and electrical connection to a self-contained portable signal-generator unit. The signal-generator unit is mounted to the cast only for periods of therapeutic treatment, and is removably mounted to a less-portable charging unit in intervals between periods of therapeutic treatment. Similarly, U.S. Pat. No. 4,998,532 to Griffith, entitled “Portable Electro-Therapy System”, teaches a portable non-invasive apparatus for electro-therapeutic stimulation of tissue and bone healing, worn or carried by a patient. U.S. Pat. No. 6,321,119 to Kronberg, entitled “Pulsed Signal Generator For Bioelectric Stimulation And Healing Acceleration”, teaches a pulsed signal generator for various biomedical applications, including electrical stimulation of fracture healing, treatment of osteoporosis, strengthening of freshly-healed bone after removal of a cast or other fixation device, and iontophoresis. U.S. patent application No. 20020016618 to Da Silva, et al., entitled “Integrated Cast And Muscle Stimulation System”, teaches a device that allows electrical stimulation to an anatomical site that is covered by a cast. The electrode is applied to achieve a desired physiological response (e.g., bone growth), treatment of pain, or the prevention of muscle atrophy.
Electrical stimulation treatments of injured tissue situated underneath a cast typically last 3–6 weeks, and may be significantly longer in some cases. Hence, all such systems would benefit from an electrode in which the mechanical and electrical performance is sustained, even under the harsh conditions beneath the surface of the cast.
It would be highly advantageous, therefore, to have a surface electrode for electrical stimulation of tissue having sustained mechanical and electrical performance over long-term storage and use, so as to enable transcutaneous electrical communication in a safe, reliable, and effective manner, even under difficult topical and ambient conditions.